1. Field of the Invention
The present invention relates to a rotary transfer system, and more particularly to a rotary transfer system for automatically changing the attitude of a workpiece-supporting pallet between horizontal and vertical directions, for example, and for feeding a workpiece supported on the pallet along an annular feed path while the workpiece is machined successively by radially arranged machine tools.
2. Description of Background Art
There is known a line production process in which a workpiece fed along a machining line is machined successively by different machine tools that are disposed along the machining line. A workpiece such as an engine cylinder head, for example, is required to be machined in many different steps. A machining line for producing such a workpiece requires machine tools to be spaced apart so that they will not interfere with each other during operation. Therefore, the overall production line including the machining line is considerably long, with the results that it is difficult effectively to utilize the space in the production factory, and the line production process is not efficient. Workpieces are fed along the machining line while they are mounted respectively on pallets. Therefore, as many pallets are used as the number of workpieces to be machined. If many workpieces of different types are to be machined, then it is necessary to provide as many pallets designed specifically to meet the configurations and dimensions of these workpieces. Use of such pallets is however not economical. After a workpiece has completely been machined, the pallet from which the workpiece is removed is moved back to a position where a new workpiece is to be set on the pallet. Therefore, a return conveyor or the like must be provided to return the pallet to the loading area.
Japanese Utility Model Publication No. 53-42847 discloses a rotary index system including a rotatable index table on which a plurality of rotatable jigs are mounted at certain angular intervals. With workpieces clamped respectively on the jigs, the index table is rotated in angular increments, and at the same time the jigs are rotated through desired angles.
If the rotary index system is incorporated in a production line, it is necessary that workpieces delivered along the production line be changed in attitude and paced on the respective jigs, and, after the workpieces have been machined, they be changed in attitude again and transferred back onto the production line.
More specifically, the workpieces are held in a horizontal attitude while they are being conveyed along the production line, but the workpieces have to be held in a vertical attitude while they are being machined. Accordingly, it is necessary to change the attitude of workpieces between horizontal and vertical directions between the production line and the rotary index system. It is considerably tedious and time-consuming for the operator to change the workpiece attitude manually.
Various devices have been proposed to change the workpiece attitude automatically. One such device is disclosed in Japanese Laid-Open Patent Publication No. 50-26177, for example. The disclosed device includes a loading device for change shifting a workpiece between horizontal and vertical attitudes by rolling the workpiece 90.degree. over in a vertical plane, and also for delivering the workpiece in the vertical attitude to a machine tool.
After the workpiece has been rolled over by 90.degree., it is delivered from the loading device to one of various machine tools. In practice, it takes a long time to load the workpiece onto the machine tool. Therefore, it has been difficult to machine the workpiece efficiently.
When the index table of the rotary index system disclosed in Japanese Laid-Open utility model Publication No. 53-42847 rotates, the jigs are rotated to respective angular positions by gears meshing with a gear mounted on the index table. The index table should therefore be rotated in highly accurate angular increments or steps. If the index table was rotated through incorrect angles, the angular positions of all the jigs would become improper, and the workpieces set on the respective jigs would not be machined as desired. It is however not easy to angularly position the index table highly accurately. Difficulty has been experienced in incorporating the rotary index system in a rotary transfer system which positions a workpiece successively with respect to a plurality of radially arranged machine tools.
Another proposed rotary index system includes a single main gear mounted coaxially on a rotatable index table and rotatable independently of the rotatable index table, and a plurality of jigs mounted on the index table at angular intervals and having respective subgears meshing with the main gear (see Japanese Laid-Open Patent Publication No. 54-82782).
With this rotary index system, workpieces supported on the respective jigs are positioned with respect to respective machine tools when the single main gear is rotated to rotate the subgears which rotate the jigs. Controlling the rotation of the main gear to position the workpieces is however quite a complex process. In real operation, different areas of the workpieces are machined by the machine tools. Consequently, it is almost impossible to index the workpieces to respective exact angular positions with respect to the respective workpieces.
When a workpiece is to be machined by a machine tool, the workpiece must be securely positioned with respect to the workpiece. Japanese Laid-Open Utility Model Publication No. 59-78042 discloses an arrangement for indexing a workpiece with respect to a machine tool. More specifically, a pallet supporting a workpiece is delivered along a straight guide rail successively through machining stations. After the guide rail and the pallet have been pulled in unison toward a jig by means of a clamp, the jig is rotated through a certain angle to index the workpiece to a desired angular position with respect to a machine tool.
This indexing system, however, cannot easily be combined with a rotary transfer system because of the difference between the straight feed path of the indexing system and the annular feed path of the rotary transfer system. More specifically, the straight guide rail for delivering the pallet is integrally joined to the clamp. If the clamp was positioned on an arcuate guide rail of the annular feed path, then the pallet traveling along the arcuate guide rail would interfere with the clamp. It would not be possible to combine the clamp directly with the rotary transfer system If the clamp is to be somehow put in the rotary transfer system, then a special mechanism is needed to avoid physical interference between the pallet and the clamp. The resulting mechanism would however be complex and expensive. A feed device for feeding the pallet linearly cannot be used as an annular feed device on the rotary transfer system.